Method for extracting ethane from an initial natural gas stream and corresponding plant

ABSTRACT

A method includes:recovering and compressing a head stream coming from a separation column, to form a stream of compressed purified natural gas;liquefying the stream of compressed purified natural gas in a liquefaction unit to form a stream of a pressurized liquefied natural gas;flash expanding of the stream of pressurized liquefied natural gas and recovering in a storage;recovering and compressing of a flow of flash gas coming from the expanding;separating the flow of compressed flash gas (132) into a fuel stream and a recycle stream;cooling and expanding the recycle stream, then introducing the cooled and expanded recycle stream at a head stage of the separation column.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage filed under 35 U.S.C. § 371, based on International PCT Application No. PCT/EP2021/081135, filed on Nov. 9, 2021, which claims priority to French Application FR 2011521 filed on Nov. 10, 2020 in the French Patent Office. The entire contents of these applications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a method for extracting ethane from a stream of initial natural gas, comprising the following steps:

-   -   cooling the stream of initial natural gas in at least one first         upstream heat exchanger, to form a stream of cooled natural gas;     -   separation of the stream of cooled natural gas into a liquid         flow and a gas flow;     -   expansion of the liquid flow and introduction of at least one         stream coming from the liquid flow into a column to separate         methane and C2+ hydrocarbons, at a first level;     -   formation of a turbine feed stream from the gas flow;     -   expansion of the turbine feed stream in a dynamic expansion         turbine and introduction of the expanded stream coming from the         dynamic expansion turbine into the separation column at a second         level,     -   introduction of a bottom stream rich in C2+ hydrocarbons         recovered from the separation column into a fractionation         column, and recovery from the fractionation column of a flow of         ethane;     -   recovery and compression of at least a part of a head stream         coming from the separation column, for forming a stream of         compressed purified natural gas;     -   liquefaction of the compressed purified natural gas stream in a         liquefaction unit, for forming a stream of pressurized liquefied         natural gas;     -   flash expansion of the stream of pressurized liquefied natural         gas and recovery, in a storage, of expanded liquefied natural         gas;     -   recovery of at least one flow of flash gas coming from the         expansion of the stream of pressurized liquefied natural gas;     -   compression of the or each flow of flash gas.

Such a method is intended in particular for extracting ethane and C3+ hydrocarbons from an initial natural gas, while producing a pressurized treated natural gas, which is then liquefied before being expanded for storage.

Ethylene, ethane, propylene, propane and heavier hydrocarbons can be extracted from gases such as natural gas, refinery gas and synthetic gases obtained from other hydrocarbon sources such as coal, crude oil, naphtha.

Natural gas usually contains a majority of methane and ethane (e.g., methane and ethane make up at least 50 mol % of the gas). Natural gas can also contain more negligible amounts of heavier hydrocarbons such as propane, butanes, pentanes and also hydrogen, nitrogen and carbon dioxide.

The invention described herein relates more particularly to the recovery, from natural gas, of ethane, propane and heavier hydrocarbons. In addition to the fact that heavy hydrocarbons in natural gas, such as ethane, propane and butane, can be highly valued by marketing same separately with a high purity, said hydrocarbons might condensate during transport or freeze in liquefaction exchangers (for the heaviest of hydrocarbons).

The above can lead to incidents, such as the arrival of liquid plugs in transport facilities or shut-downs of the liquefaction plant to unclog frozen exchangers.

U.S. Pat. No. 6,578,379 describes a very efficient method for recovering ethane and propane from a stream of natural gas. Such a method generally operates in a very efficient manner, in particular for obtaining a very high extraction (e.g. greater than 99 mol %) of the ethane contained in the natural gas feed, while minimizing energy consumption.

In order to obtain such extraction yields, it is known how to use a flow very depleted in ethane at the main reflux, namely for the highest reflux of the methane and ethane separation column.

To this end, a recirculation stream is taken from the recompressed gas coming from the head of the separation column for methane and ethane. The recirculation stream is cooled in countercurrent with regard to the gas coming from the column head and is then expanded to form the main reflux introduced at the column head.

However, under certain operating conditions, the quality of the main reflux is likely to deteriorate in terms of temperature and/or composition.

E.g., if the main reflux becomes depleted in methane, the ethane separation rate in the column decreases, and the quality of the head stream produced at the head of the column deteriorates further, aggravating the methane depletion of the main reflux. A “snowball” effect occurs, leading to a significant decrease in the ethane extraction rate. Such can be the case in particular if liquid is entrained on the upper plates of the column.

One aim of the invention is to provide a flexible and very efficient method for extracting ethane and C3+ hydrocarbons from an initial stream of natural gas, wherein the ethane extraction rate is not at all or only slightly affected when fluctuations occur in the quality of the head of the separation column.

SUMMARY OF THE INVENTION

To this end, the subject matter of the invention is a method of the aforementioned type, characterized by the following steps:

-   -   separation of the flow of compressed flash gas into a fuel         stream and a recycle stream;     -   cooling and at least partial expansion of the recycle stream,         then introduction of the cooled and expanded recycle stream at a         head stage of the separation column.

The method according to the invention can comprise one or a plurality of the following features, taken individually or according to any technically possible combination:

-   -   the concentration of methane of the recycle stream is greater         than 90 mol %, in particular greater than 95 mol %;     -   the introduction of the recycle stream takes place at a first         stage starting from the top of the separation column;     -   the recycle stream is introduced and cooled in the first heat         exchanger by heat exchange with the head stream coming from the         separation column;     -   it includes the separation of the gas flow into the turbine feed         stream, introduced into the dynamic expansion turbine, and into         a reflux stream, introduced into the separation column, lower         than the recycle stream, after cooling in a second upstream heat         exchanger and static expansion of the reflux stream;     -   the cooling of the recycle stream involves flowing the flow of         the recycle stream through the second heat exchanger;     -   the expansion of the recycle stream involves flowing the flow of         the recycle stream through a static expansion valve;     -   at least part of the compressed purified head natural gas is         placed so as to exchange heat with the flow of flash gas in a         downstream heat exchanger;     -   it includes removing a recirculation stream from the stream of         compressed purified natural gas upstream of the liquefaction         unit, the recirculation stream being cooled, expanded and         introduced into the separation column;     -   the stream of pressurized liquefied natural gas is expanded in a         dynamic or static expansion component, then introduced into a         flash drum, to be separated into the expanded liquefied natural         gas introduced into the storage, and into a flow of flash gas;     -   at least one flow of flash gas is formed in the storage, when         the expanded liquefied natural gas is introduced into the         storage;     -   the stream of pressurized liquefied natural gas is introduced         directly into the storage, without flowing through a flash drum;     -   the compression of the head stream coming from the separation         column is carried out in at least one first compressor coupled         to the dynamic expansion turbine and then in a compression         machine comprising successively, a second compressor, a cooler         for the gas compressed in the second compressor, and a third         compressor, for forming the stream of compressed purified         natural gas;     -   a head flow coming from the fractionation column is cooled and         partially condensed, then introduced into a head drum, the flow         of ethane being recovered at the head of the head drum, the         bottom of the head drum being reintroduced as reflux into the         fractionation column;     -   the entire gas flow resulting from the separation of the stream         of cooled natural gas forms the turbine feed stream which is         sent to the dynamic expansion turbine without separation.

The invention also relates to a plant for the extraction of ethane from a stream of initial natural gas, comprising:

-   -   at least one first upstream heat exchanger suitable for cooling         the stream of initial natural gas, to form a stream of cooled         natural gas;     -   a separator to separate the stream of cooled natural gas into a         liquid flow and a gas flow;     -   a liquid flow expansion component;     -   a methane and C2+ hydrocarbons separation column and a system to         introduce at least one stream coming from the expanded liquid         flow into the separation column at a first level;     -   a system to form a turbine feed stream from the gas flow;     -   a dynamic expansion turbine suitable for expanding the turbine         feed stream and a system to introduce the expanded stream coming         from the dynamic expansion turbine into the separation column at         a second level;     -   a fractionating column, a system to introduce a bottom stream         rich in C2+ hydrocarbons from the separation column into the         fractionating column, and a system to recover a flow of ethane         from the fractionating column;     -   a system to recover and compress at least a portion of a head         stream coming from the separation column so as to form a stream         of compressed purified natural gas;     -   a liquefaction unit for the stream of compressed purified         natural gas suitable for forming a stream of pressurized         liquefied natural gas;     -   a flash expansion system for the stream of pressurized liquefied         natural gas and a recovery storage for expanded liquefied         natural gas;     -   a system to recover at least one flow of flash gas coming from         the expansion of the stream of pressurized liquefied natural         gas;     -   a system to compress the or each flow of flash gas,         characterized by:     -   a system to separate the flow of compressed flash gas into a         fuel stream and a recycle stream;     -   a system to cool and at least a partial expand the recycle         stream, and to introduce the cooled and expanded recycle stream         at a head stage of the separation column.

DESCRIPTION OF THE DRAWINGS

The invention will be better understood upon reading the following description, given only as an example and made in reference to the enclosed drawings, wherein:

FIG. 1 is an flow chart showing a first installation for implementing a first method for extracting ethane according to the invention;

FIG. 2 is a chart similar to the diagram shown in FIG. 1 , of a second installation for using second method for extracting ethane according to the invention;

FIG. 3 is a chart similar to the diagram shown in FIG. 1 , of a third installation for the implementation of a third method for extracting ethane according to the invention;

FIG. 4 is a chart similar to the diagram shown in FIG. 1 , of a fourth installation for implementing a fourth extraction method according to the invention;

FIG. 5 is a chart similar to the diagram shown in FIG. 2 , of a fifth installation for implementing a fifth extraction method according to the invention.

DETAILED DESCRIPTION

Throughout hereinafter, the same reference will be used for identifying a flow of liquid and the pipe which carries the fluid, the pressures considered are absolute pressures, and the percentages considered are molar percentages.

The methods described were modeled on a process simulator. 82% polytropic efficiencies for compressors and 86% adiabatic efficiencies for turbines, were defined.

A first installation 10 for extracting ethane according to the invention, is illustrated in FIG. 1 .

The installation 10 is intended for the simultaneous production, from a stream initial natural gas 12, of an ethane-rich flow 14, of a bottom stream 16 rich in C3+ hydrocarbons, of an expanded liquefied natural gas 18, and of a fuel stream 20, advantageously intended for being reused in the installation 10.

With reference to FIG. 1 , the installation 10 includes an ethane extraction unit 22, a liquefaction unit 24, and a flash and storage unit 26 for liquefied natural gas.

The extraction unit 22 includes first and second upstream heat exchangers 28, 30, a separating drum 32, and a column 34 for separating methane and C2+ hydrocarbons. The column 34 is herein provided with a bottom reboiler 35.

The unit 22 further includes a dynamic expansion turbine 36 coupled to a first compressor 38, a second compressor 40, each compressor 38, 40 being provided downstream with a cooler 42, 44.

The unit 22 further includes a bottom pump 46, a fractionation column 48 provided with a bottom reboiler 50 and a reflux system 52, the reflux system 52 including a cooler 54, a reflux drum 56, and a reflux pump 58.

The natural gas liquefaction unit 24 is a known unit, in particular a C3MR or a DMR unit.

In the example shown in FIG. 1 , the flash and storage unit 26 includes an expansion component 60, herein a dynamic expansion turbine, a flash tank 62, and a pump 64 for conveying liquefied natural gas to a storage 66. In a variant, the expansion component 60 is a static expansion valve.

The storage 66 is e.g. a thermally insulated storage tank.

In the present example, the flash and storage unit 26 further includes a downstream heat exchanger 68, if appropriate a suction drum 70, and a compression apparatus 72 including a plurality of compressors 74 mounted in series, separated therebetween by coolers 76.

A first method according to the invention, implemented in the installation 10, will now be described.

The initial natural gas forming the stream 12 is advantageously a dry, at least partially decarbonated, desulphurized natural gas.

The term “at least partially decarbonated” means that the concentration of carbon dioxide in the stream of initial natural gas 13 is advantageously less than or equal to 50 ppmv.

Similarly, the concentration of water is less than 1 ppmv, advantageously less than ppmv.

The concentration of sulfur-containing elements, including hydrogen sulfide, is less than 10 ppmv and advantageously less than or equal to 4 ppmv.

An example of the molar composition of the stream of initial natural gas 12 is given in the table below.

TABLE 1 Molar fraction in % Nitrogen 0.19 Methane 90.62 Ethane 6.56 Propane 2.05 i-butane 0.25 n-butane 0.23 C5+ 0.09

More generally, the molar fraction of methane in the stream of initial natural gas 12 is comprised between 75 mol % and 95 mol %, the molar fraction of C2 hydrocarbons is comprised between 3 mol % and 12 mol %, and the molar fraction of C3+ hydrocarbons is comprised between 1 mol % and 8 mol %.

The flow rate of the stream of initial natural gas 12 is e.g. greater than 2,000 kmol/h and is e.g. comprised between 2,000 kmol/h and 70,000 kmol/h, in particular equal to kmol/h.

The stream of initial natural gas 12 has a temperature close to ambient temperature, in particular between 0° C. and 40° C., herein equal to 21.5° C., and a pressure advantageously greater than 35 bar, in particular greater than 70 bar, in the present example equal to 81 bar.

The initial natural gas 12 is introduced into the first heat exchanger 28 to be cooled therein. It forms a stream 80 of cooled natural gas. The initial natural gas 12 is herein supercritical, so it is simply cooled. In a variant, the initial natural gas is not supercritical and is at least partially condensed in the first heat exchanger 28.

It has a temperature below −20° C., and in particular between −25° C. and −45° C., in particular equal to −37° C.

The stream 80 is then introduced into the separator drum 32, to be separated therein into a liquid flow 82, recovered at the bottom of the separator drum 32, and a gas flow 84 recovered at the head of the separator drum 32. The flow rate of the liquid flow 82 can be zero, in particular when the stream of cooled natural gas 80 is supercritical.

The liquid flow 82 flows through a static expansion valve 86 to form a mixed expanded phase 88. The pressure of the expanded mixed phase 88 is less than 50 bar, in particular less than 30 bar, and is e.g. equal to 28.7 bar. The expanded mixed phase 88 is introduced at a bottom level N1 of the separation column 34.

The gas flow 84 is divided into a main turbine feed stream 90 and a secondary reflux stream 92.

The molar flow rate of the turbine feed stream 90 is greater than the molar flow rate of the reflux stream 92, and in particular comprised between 5% and 25% of the molar flow rate of the reflux stream 92.

The turbine feed stream 90 is introduced into the dynamic expansion turbine 36 to be expanded therein to a pressure of less than 50 bar, in particular less than 30 bar, e.g. equal to 28.7 bar.

The dynamic expansion of the current 90 makes it possible to recover more than kW of energy, e.g. 10,865 kW of energy.

The temperature of the cooled and expanded stream 94 coming from the dynamic expansion turbine 36 is e.g. less than −70° C., in particular less than −80° C., e.g. equal to −80.8° C.

The cooled and expanded stream 94 is then introduced into the separation column 34 at a level N2 situated above level N1.

The reflux stream 92 is introduced into a static expansion valve 96 in order to be expanded therein to a pressure of less than 50 bar, in particular less than 30 bar, in particular equal to 28.7 bar. It is cooled in the second upstream heat exchanger 30 to a temperature below −80° C., in particular below −90° C., in particular equal to −95.8° C.

The expanded and cooled reflux stream is introduced into the separation column 34 at a level N3 situated above the level N2 at the head of the column 34.

The pressure of the separation column 34 is preferentially between 10 bar and 40 bar, in particular between 20 bar and 40 bar, e.g. substantially equal to 28.5 bar.

The separation column 34 produces a head stream 98. The head stream 98 is heated in the second upstream heat exchanger 30, then in the first upstream heat exchanger 28 in countercurrent with the initial natural gas 12, for forming a heated head stream 100.

The temperature of the heated head stream 100 is greater than 0° C., in particular greater than 15° C., and is e.g. equal to 17.6° C.

The heated head stream 100 is then compressed in the compressor 38 coupled to the turbine 36, then cooled in the cooler 42, for obtaining a stream at a pressure greater than 30 bar, in particular equal to 34.6 bar.

It is then recompressed in the compressor 40, then cooled in the cooler 44 in order to produce a stream of compressed purified natural gas 102 intended for the liquefaction unit 24.

The stream compressed purified natural gas 102 has a pressure greater than 60 bar, in particular greater than 80 bar, e.g. equal to 91 bar. It has a temperature greater than in particular greater than 10° C., more particularly equal to 21.5° C.

The coolers 42, 44 are herein fed by a cooling flow with a temperature of less than in particular equal to 7° C. The cooling flow can be, in particular, air or water.

The compressed purified natural gas stream 102 is rich in methane. It has a concentration of methane greater than 99.0 mol %, in particular equal to 99.1 mol %. It has a low concentration of nitrogen, in particular less than 1.0 mol %, and a low concentration of C2+ hydrocarbons, more particularly a concentration of ethane of less than 0.5 mol %, substantially equal to 0.2 mol % ethane.

The separation column 34 produces at the bottom, a bottom stream 106 rich in C2+ hydrocarbons. The stream 106 contains e.g. more than 95 mol % of the ethane contained in the initial natural gas 10, and 100 mol % of the C3+ hydrocarbons contained in said stream.

The bottom current 106 has a temperature greater than 10° C., in particular comprised between 20° C. and 30° C., e.g. equal to 23.2° C. It contains less than 1000 ppmv of carbon dioxide, preferentially between 200 ppmv and 500 ppmv of carbon dioxide, e.g. 313 ppmv of carbon dioxide. It has a concentration of methane of less than 5 mol %, e.g. between 0 mol % and 3 mol %, in particular less than 1 mol %.

The table below illustrates an example of the composition of the bottom current 106.

TABLE 2 Molar fraction in % Nitrogen 0.0 Methane 0.21 Ethane 70.5 Propane 22.8 i-butane 2.8 n-butane 2.6 C5+ 0.97

A first lateral reboiling stream 108 is extracted from the separation column 34, at a level N5 lower than level N1, e.g. located at the 20^(th) stage from the top of the separation column 34.

The first liquid reboiling stream 108 is brought to the first heat exchanger 28, to be heated therein in the heat exchanger 28 by heat exchange, in particular with the initial natural gas 12, to a temperature greater than 0° C., in particular equal to 8.25° C. The reboiling stream 108 is then reintroduced into the separation column 34 at a level N7 situated below the level N5, e.g. at the 21^(st) stage starting from the top of the column 34.

Similarly, a second liquid reboiling stream 110 is extracted from the separation column 34 at a level N7 lower than the level N6, e.g. from the 22^(th) stage from the top of the separation column 34, to be brought to the bottom reboiler 35 in order to be heated therein to a temperature above 0° C., e.g. equal to 10.7° C. An energy greater than 1 MW, e.g. equal to 4 MW, is supplied to the second liquid reboiling stream 110.

The second liquid reboiling stream 110 is then returned to the separation column 34 at a level N8 located below level N7. The level N8 is e.g. located on the 23^(th) stage starting from the top.

The bottom stream 106 is pumped into the pump 46 to be introduced at an intermediate level P1 of the fractionation column 48.

The fractionation column 48 produces, at the head, a head flow 112 containing less than 1 mol % of C3+ hydrocarbons, more particularly less than 1 mol % of propane.

The head flow 112 is partially condensed in the cooler 54, then separated in the reflux drum 56 so as to form at the top the ethane-rich stream 14, and at the bottom, a liquid reflux stream 114 reintroduced at the top of the fractionation column 48, after pumping by the reflux pump 58.

The ethane-rich flow 14 contains more than 96 mol % of the ethane contained in the initial natural gas 12. It contains more than 97 mol % of ethane.

The ethane-rich flow 14 is herein gaseous. In a variant (not shown), the ethane-rich flow 14 is a liquid taken from the liquid stream 114.

The C3+ hydrocarbon stream contains less than 500 ppmv of ethane, more particularly less than 100 ppmv of ethane.

The stream of compressed purified natural gas 102 is brought into the liquefaction unit 24 which produces, in a known manner, a stream of pressurized liquefied natural gas 120.

The stream of pressurized natural gas has a pressure greater than 20 bar, in particular comprised between 20 bar and 90 bar, advantageously equal to 73 bar. It has a temperature below −120° C., in particular below −130° C., and advantageously equal to −136.8° C.

The compressed liquefied natural gas 120 is introduced into the expansion component 60, herein into a dynamic expansion turbine. It is expanded to a pressure of less than 5 bar, in particular less than 2 bar, e.g. equal to 1.25 bar, to form a stream of flashed liquefied natural gas 122.

The stream of flashed liquefied natural gas 122 is introduced into a flash drum 62 to be separated therein into a stream of expanded liquefied natural gas 124 and a first flow of flash gas 126.

The expanded liquefied natural gas stream 124 is pumped into the storage tank 66 by means of the pump 64, to form the expanded liquefied natural gas 18.

The first flow of flash gas 126 is recovered at the head of the flash drum 62. It is introduced into the downstream heat exchanger 68 in order to be heated there in countercurrent with a portion of the compressed purified natural gas 102, which is reintroduced into the stream of flashed liquefied natural gas 122, upstream of the flash drum 62.

After heat exchange in the downstream heat exchanger 68, the flow of heated flash gas 130 thus formed has a temperature greater than −60° C., and in particular substantially equal to 5° C. It has a very high concentration of methane, e.g. greater than 80 mol %, e.g. greater than 85 mol %, in particular greater than 90 mol %. Such concentration is advantageously greater than 95 mol % of methane, in particular greater than 96 mol % of methane, e.g. equal to 96.46 mol % of methane.

It has a concentration of nitrogen of less than 20 mol %, e.g. less than 15 mol %, in particular less than 10 mol %. Said concentration is advantageously less than 5 mol %, in particular less than 4 mol %, e.g. substantially equal to 3.54 mol % of nitrogen.

The flow of heated flash gas 130 has a concentration of ethane of less than 50 ppmv, in particular less than 10 ppmv, e.g. equal to 5 ppmv.

After flowing through a suction flask 70, the flow of heated flash gas 130 is compressed in the compression apparatus 72 to a pressure greater than 25 bar, in particular greater than 30 bar, and e.g. equal to 60 bar in order to produce a flow of compressed flash gas 132.

The flow of compressed flash gas 132 is separated into the stream of fuel 20 and a recycle stream 134.

The fuel stream 20 is intended to be sent to the fuel gas network of the installation to supply e.g. gas turbines of the natural gas liquefaction unit 24 or the gas turbines of an electric current generation unit intended e.g. for supplying the compressor 40 or other equipment of the installation 10.

The recycle stream 134 has a pressure greater than 30 bar, in particular greater than 50 bar, e.g. equal to 58.5 bar.

It is conveyed successively into the first heat exchanger 28 and then into the second heat exchanger 30 in order to be cooled therein to a temperature below −80° C., in particular below −90° C., e.g. equal to −95.5° C.

The recycle stream 134 is then expanded in a static expansion valve 136 to a pressure of less than 50 bar, in particular less than 30 bar, e.g. equal to 28.7 bar, to be introduced into the separation column 34 at a head level N9 of the column 34, e.g. at the first stage starting from the top of the column 34. The level N9 is situated above the level N3 for introducing the expanded and cooled reflux stream.

As indicated hereinabove, the recycle stream 134 coming from the flow of flash gas 126 is very rich in methane, since the ethane remains in the liquefied natural gas 18, or is extracted successively in the separation column 34 and then in the fractionation column 48.

Thus, the composition of the reflux introduced at the head of the separation column 34 stays very rich in methane, whatever the fluctuations in the quality of the head stream 98 of the separation column 34.

The presence of the new reflux also provides operational flexibility during the implementation of the method, and also during the design phase.

It is thus possible to optimize overall the energy consumption between the ethane extraction unit 22 and the liquefaction unit 24 in order to adjust the parameters of the two units 22, 24, in order to select, as well as possible, the compressors and the driving mode thereof, as required in the two units 22, 24. Thereby, the investment costs are significantly reduced, and also the operating costs as will be seen in the example described hereinbelow.

In a variant (not shown), the heated head stream 100 is compressed at the outlet of the compressor 38 coupled to the turbine 36 in a compression machine comprising two compression stages of the same power, the total power being equal to the power of the compressor 40. The compression machine includes an intermediate cooler which cools the gas between the compression stages. The arrangement thereby obtained provides a power saving of 5.8 MW.

A second installation 140 intended for implementing a second method according to the invention is shown in FIG. 2 .

The second method according to the invention is analogous to the first method according to the invention. It differs from the first method according to the invention in that the second method comprises a step of removal of a recirculation stream 142 from the stream of compressed purified natural gas 102.

The molar flow rate of the recirculation stream 142 is advantageously less than the molar flow rate of the residual stream of compressed purified natural gas 102, after removal of the recirculation stream 142, at the introduction thereof into the liquefaction unit 22.

The recirculation stream 142 has a pressure greater than 50 bar, in particular greater than 80 bar, e.g. equal to 90 bar. It is introduced successively into the first heat exchanger 28 and then into the second heat exchanger 30 in order to be cooled therein to a temperature below −90° C., preferentially below −95° C. and e.g. substantially equal to −95.4° C.

Then, the recirculation stream 142 is expanded to a pressure of less than 50 bar, in particular less than 30 bar, in particular equal to 28.7 bar, and is introduced into the separation column 34 between the recirculation stream 134 and the reflux stream 92.

A third installation 150 intended for implementing a third method according to the invention is shown in FIG. 3 .

The installation 150 differs from the first installation 10 in that it comprises a system 152 for collecting and recompressing the evaporation gases formed in the storage 66.

The collection system 152 comprises a protective drum 154 and a compression apparatus 156 including a plurality of compression stages 158 spaced two-by-two by a cooler 160.

A second flow of flash gas 162 resulting from the evaporation of the liquefied natural gas in the storage 66 is collected at the head of the storage 66 and is then introduced into the compression apparatus 156 to be compressed therein to a pressure greater than 25 bar, in particular between 26 bar and 70 bar, e.g. equal to 60 bar.

The second flow of compressed flash gas 164 thus produced is separated into the fuel stream 20 and the recycle stream 134, which is reintroduced into the separation column 34, after cooling in the heat exchangers 28, 30 and expansion in the expansion valve 136.

Advantageously, in the example shown in FIG. 3 , the installation 150 does not have an expansion component 60. The compressed liquefied natural gas 120 coming from the liquefaction unit 24 is directly introduced into the storage 66 for liquefied natural gas and is flashed in the storage 66.

A fourth installation 170 intended for implementing a fourth method according to the invention is shown in FIG. 4 .

The fourth installation 170 differs from the first installation 10 in that the storage units 66 are equipped, like the third installation 150, with a system 152 for collecting the evaporation gases.

During the implementation of the fourth method according to the invention, the first flow of compressed flash gas 132 and the second stream of compressed flash gas 164 are mixed, before the mixture is separated into the fuel stream 20 and the recycle stream 134.

Like before, the recycle stream 134 is reintroduced into the separation column 34 after flowing through the heat exchangers 28, 30, then being expanded in the static expansion valve 136.

A fifth installation 200 for implementing a fifth method according to the invention is illustrated in FIG. 5 .

The fifth method differs from the second method shown in FIG. 2 in that the entire gas flow 84 recovered from the drum 32 forms the turbine feed stream 90 sent to the dynamic expansion turbine 36, without separation.

By means of the invention which has just been described, it is possible to maintain a substantially constant composition of the reflux of the separation column 34, which prevents the snowball effect which occurs during fluctuations in the composition of the head stream 98 extracted from the separation column 34, in the absence of the supply of the recycle stream 134.

Said method is thus both simple and effective in maintaining a constant concentration of extracted ethane, without increasing the investment costs or the operating costs.

The energy consumption of the method is detailed in the following table.

TABLE 3 Prior art FIG. 1 FIG. 2 FIG. 3 Ethane recovery 97 97 97 97 rate (%) Compressor 48.105 49.175 49.261 49.397 power 36 (MW) Compressor power 197.471 159.230 172.732 174.874 unit 26 (MW) Compressor 2.856 38.012 22.929 No power 72 (MW) compressor 72 Compressor 3.129 3.127 3.129 25.978 power 156 (MW) LNG flow produced 778.486 778.702 778.616 778.482 at tank (ton/h) Total power 251.563 249.544 248.051 250.250 consumption (MW) Specified power 323.1 320.5 318.8 321.5 (kWh/ton)

As indicated in the above-table, the total power consumed in the presence of a reflux generated from the recycle stream 134 represents a significant reduction of the power consumed and the specified power divided by the flow-rate of the liquefied natural gas produced by the installation. 

1. A method to extract ethane from a stream of initial natural gas, comprising: cooling a stream of initial natural gas in at least one first upstream heat exchanger, to form a stream of cooled natural gas; separating the stream of cooled natural gas into a liquid flow and a gas flow; expanding the liquid flow and introducing at least one stream coming from the liquid flow at a first level into a methane and C2+ hydrocarbons separation column; forming a turbine feed stream from the gas flow; expanding the turbine feed stream in a dynamic expansion turbine to form an expanded stream and introducing the expanded stream coming from the dynamic expansion turbine into the separation column at a second level, introducing a bottom stream rich in C2+ hydrocarbons recovered from the separation column into a fractionation column, and recovering a flow of ethane from the fractionation column; recovering and compressing at least a part of a head stream coming from the separation column, to form a stream of compressed purified natural gas; liquefying the stream of compressed purified natural gas in a liquefier, to form a stream of a pressurized liquefied natural gas; flash expanding the stream of pressurized liquefied natural gas and recovering expanded liquefied natural gas in a storage; recovering at least one flow of flash gas coming from the flash expanding of the stream of pressurized liquefied natural gas; compressing the at least one flow of flash gas to form a compressed gas; separating the flow of compressed flash gas into a fuel stream and a recycle stream; at least partial cooling and expanding of the recycle stream to form a cooled and expanded recycle stream, followed by introducing the cooled and expanded recycle stream at a head stage of the separation column.
 2. The method according to claim 1, wherein a concentration of methane in the recycle stream is greater than 90 mol %.
 3. The method according to claim 1, wherein introducing the cooled and expanded recycle stream at a head stage of the separation column is carried out at a first level starting from the top of the separation column.
 4. The method according to claim 1, comprising introducing the recycle stream in the first heat exchanger and cooling the recycle stream by heat exchange in the first heat exchanger with the head stream coming from the separation column.
 5. The method according to claim 1, comprising separating the gas flow into the turbine feed stream introduced into the dynamic expansion turbine, and into a reflux stream introduced into the separation column, at a level lower than a level of introduction of the recycle stream, after cooling the reflux stream in a second upstream heat exchanger and static expansion of the reflux stream.
 6. The method according to claim 5, wherein cooling the recycle stream includes flowing the recycle stream through the second heat exchanger.
 7. The method according to claim 1, wherein the expanding the recycle stream includes flowing the recycle stream through a static expansion valve.
 8. The method according to claim 1, comprising placing at least a part of the compressed purified head natural gas in heat exchange with the flow of flash gas in a downstream heat exchanger.
 9. The method according to claim 1, comprising removing a recirculation stream from the stream of compressed purified natural gas, upstream of the liquefaction unit, the recirculation stream being cooled, expanded, and introduced into the separation column.
 10. The method according claim 1, comprising expanding the stream of pressurized liquefied natural gas a dynamic or static expander, followed by introducing an expanded expanding stream of pressurized liquefied natural gas into a flash drum to be separated into the expanded liquefied natural gas introduced into the storage, and the at least one flow of flash gas.
 11. The method according to claim 1, comprising forming the at least one flow of flash gas in the storage when the expanded liquefied natural gas is introduced into the storage.
 12. The method according to claim 11, comprising directly introducing the stream of pressurized liquefied natural gas into the storage, without flowing through a flash drum.
 13. The method according to claim 1, wherein compressing the head stream coming from the separation column is carried out in at least one first compressor coupled to the dynamic expansion turbine and then in a compression machine comprising successively a second compressor, a cooler for the gas compressed in the second compressor, and a third compressor, to form the stream of compressed purified natural gas.
 14. The method according to claim 1, comprising cooling and partially condensing a head flow coming from the fractionation column to form a cooled and at least partially condensed head flow and then introducing the cooled and at least partially condensed head flow into a head drum, recovering the flow of ethane at the head of the head drum, the bottom of the head drum being reintroduced as reflux into the fractionation column.
 15. The method according to claim 1, comprising forming the turbine feed stream which is sent to the dynamic expansion turbine without separation from an entire gas flow coming from the separation of the stream of cooled natural gas.
 16. An ethane extraction installation to extract ethane from a stream of initial natural gas, comprising: at least one first upstream heat exchanger configured to cool a stream of initial natural gas, to form a stream of cooled natural gas; a separator to separate the stream of cooled natural gas into a liquid flow and a gas flow; a liquid flow expander; a methane and C2+ hydrocarbons separation column and an inlet to introduce at least one stream coming from the expanded liquid flow into the separation column, at a first level; a dynamic expansion turbine configured to expand a turbine feed stream formed from the gas flow, configured to form an expanded stream and an inlet to introduce the expanded stream from the dynamic expansion turbine into the separation column at a second level, a fractionation column, an inlet to introduce a bottom stream rich in C2+ hydrocarbons coming from the separation column into the fractionation column, and an outlet to recover a flow of ethane from the fractionation column; an outlet to recover at least a part of a head stream coming from the separation column and a compressor to compress the at least a part of a head stream coming from the separation column, to form a stream of compressed purified natural gas; a liquefier configured to liquefy the stream of compressed purified natural gas to form a stream of pressurized liquefied natural gas; a flash expander of the stream of pressurized liquefied natural gas to form a flash expanded liquefied natural gas and a storage to recover the flash expanded liquefied natural gas; an outlet to recover at least one flow of flash gas from the flash expander; a compressor of the at least one flow of flash gas; a separator of the flow of compressed flash gas into a fuel stream and a recycle stream; a cooler/expander to cool and at least partially expand the recycle stream and form a cooled and expanded recycle stream, and to introduce the cooled and expanded recycle stream at a head stage of the separation column. 